The present invention relates to a multilayer birefringent interference polarizer, and more particularly to a multilayer coextruded polymeric device which can be designed to polarize selected wavelengths of light by constructive optical interference.
Birefringent polarizers are generally known in the art and have been used in the past to polarize and filter selected wavelengths of light. For example, birefringent polarizers may be used to reject (reflect) specific polarized narrow wavelength ranges while transmitting the remainder of the incident light, to reduce glare from other light sources, and to act as beam splitters.
Many naturally occurring crystalline compounds act as birefringent polarizers. For example, calcite (calcium carbonate) crystals have well known birefringent properties. However, single crystals are expensive materials and cannot be readily formed into the desired shapes or configurations which are required for particular applications. Others in the art, such as Makas, U.S. Pat. No. 3,438,691, have fabricated birefringent polarizers from plate-like or sheet-like birefringent polymers such as polyethylene terephthalate incorporated into an isotropic matrix polymer.
In many instances, polymers can be oriented by uniaxial stretching to orient the polymer on a molecular level such as taught by Rogers et al, U.S. Pat. No. 4,525,413. Multilayer optical devices comprising alternating layers of highly birefringent polymers and isotropic polymers having large refractive index mismatches have been proposed by Rogers et al. However, the Rogers et al device requires the use of specific highly birefringent polymers having certain mathematical relationships between their molecular configurations and electron density distributions.
Accordingly, there remains a need in the art for birefringent interference polarizers which can be readily produced using existing techniques and readily available materials. Further, there still exists a need in the art for bireftingent interference polarizers which absorb little light. Further, the need exists in the art for birefringent polarizers which can be fabricated to polarize light of specific wavelengths as desired.
The present invention meets that need by providing a birefringent interference polarizer in the form of a multi-layered sheet or film which may be fabricated from readily available materials using established coextrusion techniques. The polarizer of the present invention has a level of light absorption near zero and can be fabricated to polarize and reflect light of specific wavelengths while transmitting light of other wavelengths. The polarizer will also polarize the transmitted light at those wavelengths, while the remainder of the transmitted light remains unpolarized.
Reference to polarizers, polarized light, and polarization as used herein refers to a condition of light in which the transverse vibration of the rays assume different forms in different planes. Polarization, as used herein, includes the nonequal reflection of light in orthogonal planes and encompasses elliptical and circular polarization of light as well as plane polarization. By xe2x80x9clightxe2x80x9d, we mean not only light in the visible spectrum, but also ultraviolet and infrared light. When the plane of orientation of the polymeric materials is discussed herein, we are referring to the directions of orientation of the polymeric materials due to uniaxial or biaxial stretching of the materials in the x and/or y direction to define the polarizing effect of the materials. In other contexts, reference to the plane that light enters or impinges upon the layers of polymeric materials is a plane normal to the major surfaces of-the layers (i.e., the z direction), unless otherwise indicated.
In accordance with one aspect of the present invention, a birefringent interference polarizer is provided comprising multiple alternating oriented layers of at least first and second polymeric materials having respective nonzero stress optical coefficients which are sufficiently different to produce a refractive index mismatch between the first and second polymeric materials in a first plane which is different from the refractive index mismatch between the first and second polymeric materials in a second plane normal to the first plane.
The birefringent polarizer of the present invention may also comprise three or more alternating layers of diverse polymeric materials. For example, a three layer pattern of repeating units ABCBA may be used, where the B unit is a copolymer or miscible blend of the A and C repeat units. In some instances, the B layer may not only contribute to the light polarization properties of the invention, but also act as an adhesive layer to bond the A and C layers together.
Also, the third polymer layer may be found as a surface or skin layer on one or both major exterior surfaces for an ABABAB repeating body or as an interior layer. The skin layer may be sacrificial, or may be permanent and serve as scratch resistant or weatherable protective layer. Further, such skin layers may be post applied to the polarizer after coextrusion. For example, a skin layer may be applied as a sprayed on coating which would act to level the surface of the polarizer to improve optical properties and impart scratch resistance, chemical resistance and/or weatherability. The skin layer may also be laminated to the multilayered polarizer. Lamination is desirable for those polymers which are not readily coextrudable.
In one embodiment of the invention, the first and second polymeric materials have substantially equal refractive indices when unoriented. The refractive index mismatch develops in the plane of orientation when the materials are stretched. In another embodiment, the first and second polymeric materials have differing refractive indices when unoriented. Orienting the polymers by stretching causes the mismatch between respective refractive indices in one of the planes to decrease, while the mismatch in the other plane is maintained or increased. The polarizer may be uniaxially or biaxially oriented.
In a preferred form of the invention, the first polymeric material has a positive stress optical coefficient, while the second polymeric material has a negative stress optical coefficient. Preferably, the refractive index mismatch in the first plane is at least 0.03, and most preferably 0.05 or greater.
Preferably, the optical thickness of each polymeric layer is from about 0.09 micrometers to about 0.70 micrometers. Optical thickness, nd, is defined as the product of the physical thickness of the layer (d) and its refractive index (n). In a preferred form of the invention, the layers increase in thickness monotonically through the thickness of the film to produce a layer thickness gradient which reflects and polarizes a broad range of wavelengths of light.
The two polymeric materials can be any of a number of different polymers which possess nonzero stress optical coefficients which provide the necessary refractive index mismatch when the materials are oriented. By nonzero stress optical coefficient, it is meant that the refractive index of the polymer changes in either a positive or negative direction when the polymer is oriented. Isotropic materials possessing zero stress optical coefficients lack birefringence.
For example, the first polymeric material may be a polycarbonate, such as a bisphenol A based polycarbonate, or a polyethylene terephthalate, both of which possess positive stress optical coefficients. The second polymeric material may be a polystyrene which has a negative stress optical coefficient. Either generally amorphous atactic polystyrenes or more crystalline syndiotactic polystyrenes are suitable. Other suitable polymers for the second polymeric material include copolymers of styrene and acrylonitrile, copolymers of styrene and methyl methacrylate, and polyethylene naphthalate, all of which possess negative stress optical coefficients.
The polarizer of the present invention reflects and polarizes a portion of the light incident on its surface while transmitting the remainder of the incident light. During fabrication, it may be designed to transmit only a narrow range of wavelengths while reflecting a broad range, or vice versa. The polarizer of the present invention may Also be designed to reflect and polarize substantially all light incident in one plane of the device while transmitting substantially all light incident in a plane normal thereto.
In some embodiments of the invention it may be desirable to incorporate coloring agents such as dyes or pigments into one or more of the individual layers of the birefringent polarizer. This can be done to one or both of the outer or skin layers of the body, or alternatively, the coloring agent may be incorporated into one or more interior layers in the polarizer. The use of pigments or dyes permits the selective absorption of certain wavelengths of light by the polarizer. While an unpigmented or undyed multilayer film will reflect specific polarized wavelengths and transmit the remainder of incident light, pigments and dyes can be used to further control the bandwidth of reflected polarized light and the wavelength range of transmitted light. For example, all transmitted light may be absorbed by coextruding a black layer on the back side of the birefringent polarizer. Furthermore, dyes may be used to narrow the wavelength band of reflected polarized light and transmitted light by absorbing selected wavelengths.
The polymers chosen will determine the refractive index mismatch, respective stress optical coefficients, and glass transition temperatures. The number of layers, degree of orientation, layer thicknesses, and use of pigments or dyes may all be adjusted (controlled) to provide a polarizer having the desired characteristics for a particular end use. This contrasts to prior art devices which were limited both in design and polarization characteristics.
In another embodiment of the invention, a tunable birefringent interference polarizer is provided and comprises multiple alternating layers of first and second elastomeric materials having respective nonzero stress optical coefficients which are sufficiently different to produce a refractive index mismatch between the first and second elastomeric materials in a first plane which is different from the refractive index mismatch between the first and second elastomeric materials in a second plane normal to the first plane. Because the individual layers forming the polarizer are elastomers, the polarizer variably polarizes wavelengths of light dependent upon the degree of elongation of the elastomers. Additionally, because the layers are elastomers, the polarizer is tunable and reversible as the device is returned to a relaxed state.
The present invention also provides a method of making a birefringent interference polarizer comprising the steps of coextruding at least first and second polymeric materials having respective nonzero stress optical coefficients in multiple layers. The layers may be stretched to orient the polymeric materials and produce a refractive index mismatch in a first plane which is different from the refractive index mismatch between the first and second polymeric materials in a second plane normal to the first plane. While many polymer combinations can be stretched at temperatures above the glass transition temperature but below the melting temperature of the polymers, some polymer combinations can be xe2x80x9ccold drawn,xe2x80x9d where one or more of the polymers can be stretched at a temperature below its glass transition temperature.
In one embodiment of the invention, the first and second polymeric materials have substantially equal refractive indices when unoriented, with a refractive index mismatch in one plane developing upon orientation. In another embodiment, when oriented, the first and second polymeric materials have substantially equal refractive indices in one of the first and second planes, but there is a refractive index mismatch in the other plane. The orientation of the polymeric materials may be either uniaxial or biaxial.
Preferably, the refractive index mismatch in the first plane is at least about 0.03, and most preferably at least 0.05 or greater, with the optical thickness of each layer being from about 0.09 micrometers to about 0.70 micrometers. In one embodiment, the layers increase in thickness monotonically through the thickness of the film to provide a polarizer which reflects a broad range of wavelengths. In a preferred form of the invention, the first polymeric material has a positive stress optical coefficient, and the second polymeric material has a negative stress optical coefficient.
Accordingly, it is an object of the present invention to provide a birefringent interference polarizer, and method of making, which may be fabricated from readily available materials, using established coextrusion techniques, to include having a level of light absorption near zero and be fabricated to reflect and polarize light of specific wavelengths while transmitting light of other wavelengths. This, and other objects and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.